A variety of thermoelectric devices are known in the art, using thermoelectric materials for the production of electricity or for cooling and heating applications. Thermoelectric devices can have distinct advantages in many applications. For example, an electric power generator based on thermoelectric materials does not use moving parts like conventional power generators. This feature significantly enhances the reliability of the thermoelectric devices by avoiding mechanical wear of the moving parts and corresponding failure. This further reduces the cost of maintenance. Thermoelectric devices also allows operation in hostile environments such as in high temperature conditions (e.g., 900° C.) without human attendance. The unique properties of thermoelectric materials also make the thermoelectric devices environmentally friendly, i.e., industrial heat waste or natural heat sources can be used to generate electric power.
Thermoelectric materials are metals, semi-metals, or semiconducting materials that can convert thermal energy into electrical energy, and visa versa. The basic thermoelectric effects underlying this energy conversion are the Seebeck and Peltier effects. The Seebeck effect is the phenomenon underlying the conversion of heat energy into electrical power and is used in thermoelectric power generation. When a thermoelectric material is subjected to a temperature differential, the Seebeck effect produces an open voltage across the material, which can be used to drive an external load. The complementary effect, the Peltier effect, is the phenomenon used in thermoelectric refrigeration and is related to heat absorption accompanying the passage of current through the junction of two dissimilar materials. When supplied with a voltage, thermoelectric semiconductors respond by virtue of the Peltier effect to produce a temperature differential that can heat or cool an external load.
Although the Seebeck and Peltier effects can be seen with a wide variety of materials, the magnitude of the effect (the Seebeck coefficient, S=dV/dT, where V is voltage and T is temperature) is so low with most materials as to have no practical application. Only certain materials have been found to produce significant thermoelectric effects. Some thermoelectric materials are semiconducting or semi-metallic. These materials conduct electricity by using two types of carriers: electrons and “holes.” When one atom in a crystal is replaced by another atom with more valence electrons, the extra electrons from the substituting atom are not needed for bonding and can move around throughout the crystal. Such materials are called “n-type” semiconductors. On the other hand, if an atom in the crystal is replaced with another atom having fewer valence electrons, one or more bonds are left vacant and thus positively charged “holes” are produced, which may be conducting carriers. Such materials are called “p-type” semiconductors.
In its simplest form a thermoelectric module can be constructed around a single semiconductor pellet which is soldered to electrically-conductive material on each end (usually plated copper). Such a module is depicted in FIG. 1a. It is important to note that the heat moves in the direction of charge carrier movement throughout the circuit. In this example, an n-type semiconductor material is used to fabricate the pellet so that electrons (with a negative charge) will be the charge carrier employed to create the thermoelectric effect. P-type semiconductor pellets can also be employed, as shown in FIG. 1b. 
While a simple thermoelectric device might be made with a single semiconductor pellet such a device cannot convert an appreciable amount of thermal energy to electricity. In order to provide useful thermoelectric capacity, multiple pellets are used together. Thus, a thermoelectric converter consists of a number of alternate n- and p-type semiconductor elements, which are connected electrically in series by metal interconnects, and sandwiched thermally in parallel between two electrically insulating but thermally conducting ceramic plates, to form a module. If a temperature gradient is maintained across the module, electrical power will be delivered to an external load and the device will operate as a generator. Conversely, when an electric current is passed through the module, heat is absorbed at one face of the module and rejected at the other face, and the device operates as a heat pump.
The efficiency of the thermoelectric materials in such devices is often characterized by a thermoelectric figure of merit, ZT. ZT is a dimensionless parameter and is conventionally defined as:ZT=(S2σ/κ)T; where S is the thermopower or Seebeck coefficient, σ the electrical conductivity (S/cm), κ the thermal conductivity (W/m-K) and T the temperature (K). The figure of merit represents the coupling between electrical and thermal effects in a material. The larger the ZT, the higher the energy conversion efficiency of a thermoelectric material. An efficient thermoelectric material should have a large Seebeck coefficient, high electrical conductivity, and low thermal conductivity.
Thermoelectric materials such as alloys of PbTe, and Bi2Te3, BiSb, and others of the formula Bi2−xSbxTe3−ySey, are well known in the art. However, the efficiency of thermoelectric devices made using these materials is relatively low, at approximately five to eight percent energy conversion efficiency. For the temperature range of −100° C. to 1000° C., the maximum ZT of such thermoelectric materials is limited to values of about 1. Furthermore, for the materials such as PbTe and Bi2Te3, the number of isostructural compounds and the possibility to optimize their properties for maximum performance at different temperatures of operation are limited.
Accordingly, an object of recent research has been to find new materials with enhanced thermoelectric properties. Several classes of materials have been investigated, including complex ternary and quaternary chalcogenides, ternary skutterudites, half-Heusler alloys, ternary metal oxides, intermetallic clathrates, and pentatellurides. Such materials have been described in the following references: Kanatzidis, Semicond Semimet, 69, 51-100, (2000); Sales et al., Science 272 (5266): 1325-1328, (1996); Poon, Semicond Semimet 70, 37-75, (2001); Terasaki et al., Jpn J Appl Phys 2 40 (1AB): L65-L67, (2001); Sales et al., J Solid State Chem 146, 528-532 (1999); Nolas et al., Semicond Semimet 69, 255-300, (2001); Latturner et al., Solid State Chem 151, 61-64 (2000); and Tritt et al., Semicond Semimet 70, 179-206, (2001). In another approach, artificial superlattice thin film structures grown from chemical vapor deposition of Bi2Te3/Sb2Te3, and molecular beam epitaxy (MBE) of PbSe0.98Te0.02/PbTe have been described with significantly enhanced ZTs than their bulk counterparts. Such materials are described in the following references: Venkatasubramanian et al., J Cryst Growth 170, 817-821, (1997); Harman et al., J Electron Mater 25, 1121-1127 (1996); Beyer et al., Appl Phys Lett 80, 1216-1218 (2002); Venkatasubramanian et al., Nature 413, 597-602, (2001); and Harman et al., J Electron Mater 29 (1): L1-L4 (2000). Nevertheless, an even more desirable breakthrough in this area would be the discovery of new reproducible compositions that would generate similar ZT values in a bulk material. This is because the vast majority of applications require bulk materials in large quantities.
Accordingly there remains a need for thermoelectric materials that have a high thermoelectric figure of merit. Use of such materials would produce thermoelectric devices with high efficiencies. Moreover, it would be desirable to have semiconductor materials that are not only good electrical conductors but have a range of band gaps to fit a wide number of applications. It would be further desirable to have materials in which the band gaps could be adjusted to give the desired band gap for the appropriate application. These materials should also be thermally and chemically stable.